Apparatus for maintaining a controlled recirculation of carrier medium through an air-swept pulverizer



Oct. 7, 1947. J HARVEY 2,428,422

APPARATUS FOR MAINTAlNING A CONTROLLED RECIRCULATION OF CARRIER MEDIUM THROUGH AN A1R-SWEPT PULVERIZER Filed April 19, 1944 3 Sheets-Sheet 1 INVENTOR Jam LHan ey BY ATTORNEY Oct. 7, 1947. J HARVEY 2,428,422

APPARATUS FOR MAINTAINING A CONTROLLED RECIRCULATION I OF CARRIER MEDIUM THROUGH AN AIR-SWEPT PULVERIZER Filed April 19, 1944 5 Sheets-Sheet 2 IN VEN TOR.

BYQ/IQIZY 85 L Han 6y A TTORNE Y Oct. 7, 1947. J HARVEY 2,428,422

' APPARATUS FOR MAINTAINING A CONTROLLED RECIRCULATION OF CARRIER MEDIUM THROUGH AN AlR-SWEPT PULVERIZER Filed April 19, 1944 5 Sheets-Sheet 3 Fig.3

INVENTOR.

cfamesl. Ham 6y Patented Oct. 7, 1947 APPARATUS FOR MAINTAINING A CON- TROLLED RECIRCULATION OF CARRIER MEDIUM THROUGH VERIZER AIR- SWEPT PUL- James L. Harvey, Fanwood, N; J assignor'to The Babcocl: & Wilcox Company, Rockleigh, N. 1., a corporation of New Jersey p Application April 19, 1944, Serial No. 531,776

The present invention relates in general to pulverizing systems comprising pulverizers of the type in which the pulverized particles are removed from'the pulverizing zone by a suitable gaseous carrier medium, such as a current of air, and more particularly to pulverizers of the character described in which the current of air or other carrier medium is caused to flow through a restricted throat or passage at the discharge side of the pulverizing zone. vIn such pulverizers the velocity of the carrier medium through the throat is desirably maintained at or above a predetermined minimum value to prevent the discharge of pulverized material downwardly through the throat, 1'

Heretofore there have been limitations in the load range over which it has been possible to operate air swept pulverizers and still maintain theratio of pulverized material to air necessary in providing a substantially uniform density of the'mixture produced? Many attempts have been made to provide a pulverized fuel firing system analogous to the uniformity of gaseous fuels wherein it would be possible to obtain a uniformity of pulverized solid material suspended in a gaseous'carrier medium throughout the changes in the quantity of that mixture obtained from the pulverizin system without corresponding manual adjustment in the pulverizing apparatus and its auxiliary equipment;

The lack of an effective and reliable density control over wide ranges in pulverizing capacity has been especially detrimental 'in connection with the pulverization of coal,"particularly where the pulverized coalladen air has been delivered to one or more points of use, as for example, to

a plurality of independently fired furnaces. The principal difliculty has been experienced in attaining uniformity of fuel supply to one furnace when variations in the withdrawal of fuel from the circulating system to other furnaces was undertaken.

The principal object of the present invention is, therefore, to provide a pulverized material preparation system'which will deliver a mixture of pulverized material and air having a predetermined density throughout a wide range in capacities.-

A further object of my'in'ventionis the provision of an air swept pulverizer wherein the flow of the carrier medium through the pulverizer threat is maintained atorabove a predetermined value independently of the introduction of makeup carrier medium to thepulverizer.

A further object of the invention is topermit anincreaseln the available operating range of 3 Claims. (01. 241'33) an air swept pulverizer extending from a maximum capacity downwardly to a minimum ca- .pacity of zero pulverizer discharge by recirculating a controlled quantity of carrier medium through the pulverizer.

A further object of the invention is to provide an air swept pulverized coal system which can "be maintained in operation without the discharge of coal and air to points of ultimate use, but which, on demand, is capable of immediately supplying coal and air of predetermined readily ignitible density in accordance with that demand.

The various novelieatures of my invention are pointed out with particularit in the claims forming a part of this specification. For a better understanding of theinvention, reference should be made to the drawings and descriptive matter pertaining to the several embodiments of the invention. I

Of the drawings: Fig. 1 is an elevation, partly diagrammatic and in section, of a pulverizin system constructed in accordance with my invention;

V Fig. 2-isa similar view of my invention as embodied in a direct firing circulating system; and 1 Fig, 3 is a similarview of my invention as embodied in a modified direct fired circulating system.

The pulverizing system illustrated in Fig. 1 consists of a pulverizer I having an air-tight casing 2. A reduction gear and motor unit 3, mounted on the casing 2, drives avertical shaft Awhich extends downwardly into the casing and in turn drives a resiliently loaded upper rotary grinding ring 5 through a yoke member 6. The grinding ring 5 restsupon a circular series of rolling grinding elements such as metallic balls "5, supported on a lower stationary grinding ring 8. A feeder 9 driven by a variable speed motor, not shown, in accordance with controls hereafter described regulates the introduction of material to be pulverized from a supply pipe in to the chamber of the pulverizer. The materialto be pulverized drops on to an annular inclined baflle II at the outer side of the balls I and enters the spaces between the adjacent balls. The balls I are rolled along the lower ring 8 by the driving contact of the upper ring 5, and the material undergoes a grinding action between the adjacent contact surfaces of the balls and rings. Material which passes inwardly through the row of balls, consisting of a high percentage of finely pulverized particles, is discharged toward the center of the rings 'andballs, and is swept up by a current of carrier air passing upwardly-through an annular the exhauster'fan I3 connected by the conduit I4: 7 to the interior of the pulverizer casin 2. The air is drawn from a central mixing chamber I;

which has two air supply connections, one to a conduit I6, which provides make up-air, and a second to a conduit I'I connectedto the discharge conduit I8 from the fan I3 and thus arranged to permit the return of a stream of air-borne v fleffeci; which is .a-function of density and velocity,

pulverized material to the pulverizer.

The conduits I6 and I! are connected to the mixing chamber so that make-up air introduced through the conduit I6 and the recirculated pulverized coal and air from the conduit I! will be mixed in the chamber I5 and pass upwardly through the throat I2 as a fairly homogeneous mixture. The'flow of the stream of mixture through the throat I2 and through the mass of material normally-occurring in an operating pulverizer in the zone abovethe throat involves a pressure differential from the mixingchamber v to the pulverizer chamber as described in U. S. v

flow resistance resulting in a pressure drop or Patent 1,965,643, issued to R. M. Hardgrove, July 10, 1934. This pressure diiferentia'l is advantageously utilized to control the feeder 9 and-l thus regulate the feed of material to be pulverized and the ratio of material to air in the mixture discharged by the 'pulverizer.

The fan I3 discharges amixture of air and finely pulverized coal through the conduit I8 to a point or points of use beyond the position at mixture through the conduit I! as evidenced by the pressure drop between connections 23 and 24. Any departure from the predetermined minimum flow will therefore cause a change in the pressure drOD which will act on the diaphragm 25to move the ipiljotjvalveifi which in turn will *"introduce "the operating fluid to one or the other side of the power piston 2| causing a readjustment in the position of the control valve 20 to reestablish the predetermined flow rate through the con- 'It i's a characteristic of the upward fluid flow through the pulverizer throat I2 that its ability to prevent a downward discharge of pulverized -material through the throat depends on its inertia and varies directly as the density and as the square of the velocity of that fluid flow. The

flow of the recirculated mixture through conduit 'I'! is thus regulated to provide a predetermined minimum whichwill give an inertia efiect through 7 the pulverizer throat 1'2 which will "be suflicient to prevent a downward discharge of pulverized is material through the throat'at reduced rates of make-up air how, or without make-up air flow,

as maybe desired. "This'wilrpermit "th'e'operation 'ofth'e pulverizer 'with-an-extended capacity range down to a zero rate of material input to the pulverizer. I V V The rate of material feed to the pulverizer is regulated by "adjusting the speed "of a feeder '9, which {is controlled by a controller 54 in accord- 1 ance with changes "in' the ratio of the pressure differentialpreated "by fluid flow through'the throat and the material mass in the pulverizer as measured by a diaphragm 21 having'pressure connections 28 and 29,'to the pressure difierwhich the recirculation conduit I1 is connected thereto; The quantity of make-up air drawn into the mixing chamber by the fan will correspond to that discharged to point of use and willvary with the position of dampers I9 which are used to control the rate of discharge. The introduction of a controlled quantity of a recirculated mixture of pulverized material and air is advantageous in that it insures a minimum fluid flow through the pulverizer throat even though no "make-up air may be flowingrinto the chamber I 5 i from supply connection I6. The quantity of the recirculated mixture is regulated by control valve '20 to maintain a flow of predetermined characteristics. This regulation is accomplished by the selective introduction of a fluid under pressure toone or the other side of a'double acting piston' 2! in the cylinder 22. A fluid pilot valve 26 directs the operating fluid to one or the other side of the piston 2! in accordance with the fluid pressures imposed on the pressure sensitive dia phragm 25 byrthe differential pressures created between the two pressure connections 23 and 24 inconduit I'I resulting from the flow of recircu- -entia1 created by flow of make-up air t iOu h'a' I flow-orifice 30 as *measured by diaphragm 3Ihaving pressure connections 321and 33. The forces created by-the differentialfpressures applied :on

' "the diaphragmsZ-l and stare arranged to be in arm about the'fu'lcrum point 48 and a corree' sponding "movement of the "attached arm I!) V lated mixture therebetween. The differential pressure or pressure drop between connections 23 and 24 in the conduit will increase with an increase in the velocity-of the recirculated flow 7 or with an increase in the density of the recircullated mixture, and conversely decrease with a decrease of either of these mixture characteristics; Moreover this pressure drop will vary directly, with thedensity and directly, in accordance with, the square of the velocity of the flowing mixture; I r

In operation, the diaphragm 25 works against a restraining spring incorporated within the fluid pilot valve 26 ,which :is adjusted to give'a predetermined lllinimum, flowjof the; recirculated Z opposition to each other and transmitted to a common lever arm "45 throughv diaphragm pins and 41. 'Thus any'un'balance in the forces ex- "erted by the 'diaphragms will "cause movement of thereby making or breakingelectrical contacts "regulatingthe operation of'the feeder motor. 7 p Inasmuch as a portion o'f the pressure drop; measured by diaphragm 2Tres'ults from the pre 7 determinedfiow ofre'eirculated material through the pulverizer the effect of this portion of the itotal force exerted by the diaphragm-must be compensated to maintain a -trueratio between the make-up air flow introduced to the system through duct 16 andithe'fee'd delivered by feeder V 9. The effect of this portion of the total forces exerted on theratio :controller 54 can be compensated by adjustment of the spring-35 so that the lever 45 is moved essentially in accordance with the ratio of pressure drops re'sulting from the flow of the make-up air through the i orifice '30 and the flow of the makeuprairthrough the pulverizer. The feed of material is accordingly maintained-at a ratecproportional tothe make-up air new -independently of the flow of recirculated mixture through the pulverizer. I 7 When th'eratio;controller is adjusted to provide: a substantially uniform ratio "of pulverized, material t air zoverthe range iOf pulverizer ca pacities' the mixture flowing-through the frecircu lating conduit -.I] will be: of substantially "uniform densitya Under-suchconditionsttheicontroliregu lating the. position of valve 20 will be adjusted to provide a stream of recirculated mixture having aipredetermined constant volumetric rate of flow. Should theratio controller be adjusted to vary the ratio of pulverized material to air over the pulverizer operating range, then the density of the mixture entering the recirculating conduit l1 will also vary and under these conditions the control valve ZUWill be regulated to provide a stream of recirculated mixture having a predetermined pressur drop through conduit ll.

With a controlled stream of pulverized coal and air recirculated to a pulverizer as described herein, it. is possible to operate the pulverizing system over an operating range from the maximum capacity of the pulverizer to a predetermined minimum capacity which may be as low as zero feed of coal to: be pulverized. Moreover, thisv is accomplished with a regulation of the air to coal ratioto give a substantially uniform density of thev mixture over the entire load range. With this arrangement it is possible to operate the pulverizer without discharging any air-borne pulverized coal to points of use and to have available a supply of fuel at a predetermined density which may be utilized, at the convenience of the operators of the equipment, by the simple adjustment of a flow control valve located in the vicinity of the point of fuel utilization.

Fig. 2 illustrates my invention adapted to a direct-firing circulating system for supplying airborne pulverized coal to a plurality of separately operable furnaces. In the preparation of pulverized coal for use in systems of this type the pulverizer l is advantageously supplied with preheated air at temperatures sufficient to evaporate,.and maintain above the dew point, any excess moisture contained in the coal delivered by the: feeder 9. As shown in Fig. 2 preheated air is drawn through the duct 58 and blended with cooling air drawn through tempering air duct '51. before passing through the make-up air duct lfi-to-the pulverizer. The proper temperature of theair delivered to the pulverizer is regulated by adjusting, the valves 52 and 53 in ducts 5t and 5|... The fanl-3 discharges air-borne pulverized coaltoa conduit l8 from which a plurality of valved branch connections 36 are arranged to supply air-borne pulverized coal to burners 31 ofseparate. furnaces 38. The carrier air inrthe air-borne pulverized coal mixture is used as primary combustion air and the necessary second.- ary air for complete combustion is added at the burners from a secondary air supply system (not shown) to form the combustible fuel mixture burned in the furnaces.

As the conduit I8 is extended beyond the last position of air-borne coal discharge, the size of the conduit is advantageously reduced, as indicated at 39, to avoid excessive velocity reduction in the fluid stream which might result in. undesirable separation of pulverized coal from the carrier air within the conduit. The conduit is further extended beyond the last burner connection in a loop carrying it back to the locality of thepulverizer, where it is divided into two branch conduits 40 and H.

It is a well known characteristic of direct fired circulating systems for pulverized coal firing that the quantity of surplus air-borne coal unused at theburners will have an inverse relationship to the quantity of air-borne coal used at the burners. For'this reason, I have found itdesi'rable to divide the surplus returning air-borne coal into a. sub stantially uniform flow through branch. I! to mixing; chamber I5 of the pulverizer and to-divert the how of. the remainder through branch. 4'0. Although branch 40 is shownin the drawing as being connected torthe suction side of the fan l3, as at M, as an alternate it may be connected to the upper portion of the pulverizer above the grinding zone. A control valve 29a is located in' either branch at or branch I! to divide the stream of surplus coaland air returning toward the pulverizer. I prefer to locate this valve 200; in branch 46' due to the relatively lower. flow resistance of the how path through this branch. directly to the fan it as compared with. the flow resistance of the flow path through the branch H and through the pulverizer l to the. fan t3. For this reason the flow control by the valve 20a will be more reliable under all return flow conditions.

Preferably, as previously described with refer:- ence to Fig. 1,.the control valve 20a is regulated by a double acting pneumatic power piston. 21!? in accordance with the differential pressure created between connections 23 and Ed in branch. I'l so that the flow of the stream of coal and. air: mixiture'therethrough will be maintained at a prede termined substantially uniform rate.

A ratio controller 55a similar to that shown. in Fig. 1' regulates the rate of unpulverized coal feed to the pulverizer by controlling the speed ofthe feeder 9 in accordance with changes in theratio between the differential pressure created across the pulverizer to the differential pressure created by flow through orifice 38 in duct [6. Makeup air fi'ow through duct It will be proportional to the withdrawal of coal-laden'air from the. circulatory system to the burners 3?- and although a portion of the total differential pressureacross the: pulverizer will bedue to the substantially uniform return flow entering from branch. ll, this effect will be compensated by establishing a counter force on the controller operation through adjustment of spring 35, so that the controller regulates the feeder drive in response to the ratio of make-up air flow and the superimposed value of differential pressure created by the flow of that make-up air through the pulverizer throat 2 and the mass of pulverized coal within the pulverizer; Moreover, since the flow through throat I2 is the summation of the predetermined substantially uniform. flow of the returned air and coal from branch. H and' the fiow of make-up air from ductv It, the throat flow will vary from a predetermined minimum equalto the return flow from duct H at a minimum pulverizer rating without make-up air flow, to a maximum flow at maximum pulverize'rr rating when a corresponding maximum make-up air flow through duct 16 occurs. As an example of the use of my invention, in an installation as shown by Fig. 2, the recirculated substantially uniform stream of air and coal will be approximately 20% of the total flow through the pulverizer throat l2 when the pulverizer is oper-' ated at maximum rating, and the flow through the pulverizer throat will Vary in the ratio'pf I- to 5 between minimum and maximum pulverizer outputs. However, depending upon the operat ing range desired while consideringother factors effecting pulverizer operation, it is within the contemplated scope. of this invention to utilize areturn flow of different ratios to total thro'atfl'ow at maximum rating. For example, under certain conditions the return flow may desirably'be as low as 5%. of the-total throat flow and under other conditions a return flow of 50% may be desirable; With. the use of a return streamofi air-borne uniform.

rati .7 pulverized material of substantially uniform fl'ow characteristics, and the introduction of a variable flow'of make-up "air to a mixing chamber; with sub-sequent fiow of the mixture through a restricted throat into a pulverizer, it has been po'ssible to operate the pulverizer to discharge a stream of pulverized coal and air of commercially uniform density and to supply a plurality of V burners over a greatly extended operatingrange of the pulverizer. l 7 The term commercially uniform density is used herein to define a density, that is the ratio of weight of coal to air in the fuel mixture, which I is uniform according to density measurements now in use in industrial plants. Forexample, in an" installation 'of the type illustrated in 'Fig; 2 the'selected ratio of coal to air was one pound of coal to 3.75 pounds of air and the density vari- 7 be confirmedby gas analysis or th products of ations were estimated to be of the order of percent with the operation considered to be Fig.3 illustrates a modification of my invention wherein one or more pulverizer units of the type shown in Fig. 1 are used to supply air-borne pulverized coal to a direct fired circulating system 1 incorporating a separate constant speed fan 42; .The-use of a separate fan 42 permits the circulalion .of larger volumes of material through the circulating system than would be desirable with the pulverizer fans I 3 alone and will permit a division of the total pulverizer and' circulating system fan static pressures so that the fans selected will have higher operational efliciencies the. pulverizer and system fan drives. In this illustration the withdrawal of pulverized coal and air from the main conduit 43 through valved and thereby lower the total power consumption of branch connections 36 to burners 31 serving separate' furnaces 38 induces'a proportionate-makeup air flow in ducts l 6 to the pulverizers. As desired the pulverizers may be usedvsingly or together and moreover the pulverizers may be of dilTerent pulverizing capacities. Preferably each pulverizer discharge conduit I 8 is provided with a tightly closable valve i l in proximity to the manifold section 45 of thecirculatory conduit 43 so that each pulverizer can be isolated from'the manifold 45 when not in use. As described before, a substantially uniform flow of pulverized coal and air is recirculated to the mixing chamber 7 I5 beneath the pulverizer to establish an air flow through the annular throat I? at or above a predetermined minimum independently of the intro duction of make-up air to the pulverizer. In addition, the flow of'returned' material through the throat and pulverizer forms an artificial zero or base yalue of difierential pressure von the'ratio controller diaphragm 21; so that the regulation of the feeders will-be responsive to variations in the of the differential pressure created by make- ,up air flow'in duct 55170 the differential pressure 1 created by that make-up 'air 'ilow'in passing through the pulverizer throat and commercial installations of this character it 7 is impractical to accurately determine thedensity of an' air-borne' stream ofpulverized'coal combustionof the'air-borne fuel for indications of uniformity or variations in the density of the fuel estream. An operator skilled in'the'a'rt can determine variations in the fuel-air'ratio" beyond combustion, since this will vary with the coal -air ratiowhe'n the fuel is burned with a constant supply of 'secondary' combustion air; more, the temperatures resulting 'fromcomb'us tion of the air-borne coal witha constant supply bf sectil'ida1ry a ii Will also vary with the --fue1-air ratio? "In connection with the" commercial operation of direct fir'ed circulating systems 'suclias 'di'sclosed, I have found that gas analysesofitheproducts of combustion and simultaneous 1 recordings of the heat of combustion by thermo-' couple indications confirm visual observations that the'co'mmerciallyuniform density ofthe'fuel air mixture supplied to the furnace can be main- "tained irrespective'of the varying fuel-air deman'ds" of other furnaces supplied by thes'ame directfired circulating system. It will be noted that in the operation of rny inl vention'a substantially uniform flowof rebir} culated pulverized coal and air is reintroduced to a mixing chamber beneath the grinding elements of the pulverizer to maintain a minimum flow therethrough' and to extend the availableoperat- 'ing '-capacityrange of the pulverizer. Likewise, the substantially uniform flow of'pulverized coal and 'air recirculated to the pulverizer permits an adjustment of the ratio controller to'correlate the feed of unground coal to the pulverizer with the make-up air flow toj-the pulverizer, so that the densit of the air-borne pulverized; coal discharged by the pulverizer may be maintained withinthe limits of a commercially-uniform den? sity over all operable pulverizer capacities; ZThis density control characteristic has proven advantageous in the operation of the direct fired cir-'- culatin system of pulverized coal firing particu- V larly as applied to industrial furnace applications requiring'a high degree of accuracyf'in combustion control' In applications of this type the cone trol of heat input can be obtained by-volumetric adjustment of a flow control valve in the vicinity of each individualburner without the necessityof readjusting the pulverizer and its auxiliarylequipment .to meet the changed fuel [requirements iThus'theoperation of 'thedirect fired circulating system of pulverized coal firing equipment' has become analogous to the operation'of other fluid and gaseousfuels in the accuracy andsi 'pnit'y 'j 7 of control of heat input'to a multiplicity'of indi-i V vidu'alburnersg 7 V It will be understood 'a'rtjthat thefterm ,air is used herein generically to include any suitable gaseous or other fluent carrier medium for conveying pulverized-mate-V rial through the pulverizing system. g 7

" Iclaim:

n the mass of v pulverized coal within the pulverizer;

vj and asja confirmation of the calculated density g of the mixture, I'resort'to observations of the a commercially uniform'density by visual'obser 'ilation of the flame; iSuch visualobservations may lIIn a coal pulverizing system, the combina of rolling grinding the discharge side of said grinding, 'elementsyari air mixing chamber" below said chamber above said throat and arranged to de} liver coal laden airthr'ough'a rmake up air; to said mixing Further by those skilled in the throat, a suction 1 fan receiving coal laden air from said pulverizing 7 connecting conduit to'at leastfone pointof usei' mean f supplying I chamber, a second, 7 conduit connectionfrom thedischarg'e of said'fan 9 to said mixing chamber, a valve arranged to re ulate the flow of coal laden air through said second conduit, and means responsive to the pressure drop of coal laden air flow through said second conduit for positioning said valve.

2. In a coal pulverlzing system, the combination of an air swept pulverizer having a casing defining a pulverizlng chamber, a circular grinding surface positioned in said chamber, a plurality of rolling grinding elements in contact with said surface, means for causing relative rotational movement between said surface and said elements, means for supplying coal to the inlet side of said grinding elements, a restricted throat at the discharge side of said grindin elements, an air mixing chamber below said throat, a suction fan receiving coal laden air from said pulverizing chamber above said throat and. arranged to deliver coal laden air through a connecting conduit to at least one point of use, means for supplying make-up air to said mixing chamber, a second conduit from the discharge of said fan connected with said mixing chamber, and means for maintaining a substantially uniform flow of coal laden air through said second conduit including a valve arranged to regulate the flow of coal laden air through said second conduit, and means responsive to the pressure drop of coal laden air flow through said second conduit for positioning said valve.

3. In a coal pulverizing system, the combination of an air swept pulverizer having a casin defining a pulverizing chamber, a circular grinding surface positioned in said chamber, a, plurality of rolling grinding elements in contact with said surface, means for causing relative rotational movement between said surface and said elements, a restricted throat at the discharge side of said grinding elements, an air mixing chamber 10 below said throat, a suction fan receiving coal laden air from said pulverizing chamber above said throat and arranged to deliver coal laden air through a connecting conduit to at least one point of use, an extension conduit arranged to receive surplus coal laden air from said point of use, said extension conduit being divided into a plurality of branches, one branch being connected t said mixing chamber, and a second branch arranged to by-pass said pulverizer and opening to the inlet side of said suction fan, a valve in said second branch arranged to regulate the flow of coal laden air therethrough, and means responsive to the pressure drop of coal laden air flow through said first branch for adjusting said valve for maintaining a substantially uniform flow through said first branch.

JAMES L. HARVEY.

REFERENCES CITED The following references are of record in the file of this patent:

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